Temperature and pressure control devices for refrigeration systems are frequently formed by a support housing containing a control switch or a valve operated by an expansible chamber actuator, such as a metal bellows. The actuator is rigidly fixed to the housing with the actuator chamber communicating with a small diameter thin walled tube frequently referred to as a capillary tube. The tube and actuator chamber contain an operating fluid and the tube functions to transmit fluid pressure to the actuator from a remote location.
The support housing assembly is quite commonly attached to the refrigerant compressor with the capillary tube extending from the housing to a desired location. The capillary tubes are formed to be relatively supple so that when a control device is installed the capillary tube can be manually bent to extend it to a desired location Typical capillary tubes are formed from soft copper which is coiled by the control manufacturer for shipment device and is uncoiled and shaped as desired by the user of the control device.
Capillary tubes used in pressure controls are formed with a pressure fitting at the tube end remote from the actuator. The remote tube end is open so that the capillary tube can be connected directly to a tap in a refrigerant system. Refrigerant at system pressure is thus communicated to the actuator chamber via the capillary tube.
In temperature sensing controls, the remote capillary tube end is hermetically sealed so that the capillary tube and actuator chamber form a sealed volume containing a thermally respnnsive fluid. The capillary tube is bent around to place part of the tube in heat transfer relationship with a part or space whose temperature is to be sensed. The actuator then is expanded or retracted according to the sensed temperature.
The actuators are normally formed from a high strength metal such as stainless steel. The actuator assemblies are hermetically welded together to provide strong fluid tight joints. The capillary tubes are generally connected to the actuators by a brazed joint because of the difference in the actuator and capillary tube materials. Typically the capillary tube end is inserted in a closely conforming sleeve-like extension of the actuator, flux and brazing compound is placed about the juncture and the tube and actuator are subjected to localized heating to complete the brazing operation. The brazing operation substantially heats a section of the capillary tue adjacent the actuator causing embrittlement of the tube end section near the brazed joint. The brazed joint itself is also brittle compared to the soft copper capillary tube.
In the past the brazed joints and relatively brittle capillary tube end sections have been subject to stress fractures during shipment and handling. This was particularly true when the capillary tubes were subjected to bending stresses near the control housing.
In order to minimize these occurrences it has been common to fix a helical stainless steel spring about the capillary tube extending from the actuator, across the brazed joint and along the tube to a location beyond the heat affected section. In some control devices a relatively stiff plastic sheath was fitted about the capillary tube in place of the coil spring. The presence of these devices tended to prevent the capillary tubes from breaking as a result of being kinked or sharply bent during shipping and handling.
Another problem which arose in the field was that of fatigue fracturing of the brazed joints or the embrittled capillary tube sections due to vibrations. Vibrations were induced in the capillary tubes and brazed joints as a result of the control housings being mounted on or in association ith a refrigerant compressor. The remote portion of the capillary tube formed spring and mass systems which tended to vibrate at different rates from the compressors. The compressors operated and induced vibrations in the capillary tubes which in turn transmitted cyclic flexural stresses to the brazed joints and adjacent tube sections. Fatigue fractures sometimes resulted from these cyclic stresses. The capillary tube stiffening springs and plastic sheaths did not eliminate or substantially reduce the occurrence of such failures.
When a capillary tube fracture occurred in a temperature control, the control failed and had to be replaced. When a failure occurred in a pressure control the control failed, but even worse the refrigerant system charge was vented to atmosphere. In many cases, loss of refrigerant charge was a more serious consequence than replacement of the control.
Failure of controls due to capillary tube fatigue fracturing was a more or less random occurrence which depended in large part on how good the brazed joint was, the degree of embrittlement of the capillary tube and how the capillary tube was bent and extended from the control housing. If the free length of the capillary tube was configured so that compressor vibrations were at or about the resonant frequency of the tube, the brazed joints or the embrittled section of the capillary tube would fail after a short period of use. If the capillary tube configuration was such that compressor induced vibrations were damped, the tubes did not tend to fail readily.